How important is the photometric stability of an spectrofluorometer for measuring excitation-emission spectra of dissolved organic matter for PARAFAC analysis?
I am working on an instrument which fails its manufacturers validation test for photometric stability. The measured drift is 3-6 % per hour whereas the instrument criteria is < 2 %. We have replaced the Xe lamp and troubleshooted extensively with the supplier but have been unable to find the root of the problem. The instrument itself is fairly new and has had little use up until now. All other instrument validation tests pass.
The stability test fails if performed to the manufacturers parameters (internal high order diffraction filter disabled). Although, this filter is enabled for our DOM sample acquisitions. If i run a manual validation following the same parameters but with the filter enabled i can get an acceptable 1 % per hour drift.
Q - should i trust my manual validation (drift < 1 %) as this is similar to the specifications of the sampling parameters or should i reject the data and be looking for an alternate spectroflurometer?
thanks!
Which instrument is it? If it has a photomultiplier tube (PMT; everything except the Aqualog has a PMT), both the lamp and the PMT are subject to drift. Since you replaced the lamp, it is possible that the PMT is failing (but that would be uncommon in my experience... so far...). If you change the PMT voltage does the problem persist? If you are on an Aqualog, then there is no PMT, but there is a CCD and in some of the earlier Aqualog models, the thermoelectric cooler (if I am remembering the part correctly) on the CCD went bad - I don't remember what the symptoms of that were. Last thought is a power supply on the fritz (could have a capacitor that is dying - they degrade over time), but that is again an uncommon occurrence. Disappointing that the manufacturer didn't have a answer.
Regardless, I would make up a batch of quinine sulfate (or just filtered surface water DOM), and spend a day putting it through the instrument alternating between drift measurements and sample measurements (maybe at hour intervals) so that you can determine if it is providing consistent data. If the data are consistent, go ahead an use it, but run a standard (Quinine sulfate) every time (day) you use it and keep an eye on it.
In the end, PARAFAC is reasonably robust to small changes in peak location and noise.
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